Conventionally, there are image reading apparatuses capable of reading an original in two different ways, i.e., reading an original stationarily placed on a platen glass by moving a scanner (referred to as “stationary reading operation” hereinafter), and reading an original by a scanner at a fixed position while feeding the original along the platen glass by an automatic document feeder (referred to as “read-while-feed operation” hereinafter) are known. This read-while-feed operation can shorten the reading time of entire originals as compared to the stationary reading method because original exchange and reading can be simultaneously performed, and the image reading section need not be returned to the home position to read the next original.
In the above-described read-while-feed operation, however, since an original image is read at a fixed position on the platen glass, black streaks may be formed on the read image due to dust or dirt between the platen glass and the original or on the lower surface of the platen glass, and an original image can not be read properly. Hence, detecting of dust and/or dirt sticking to the platen glass (referred to as “dust detection”) is executed to prevent any abnormal image reading due to dust and/or dirt on the platen glass.
In detecting dust and/or dirt in an image reading apparatus, having the above arrangement, with a function of conveying an original to be read to the reading position using a conveyor belt, normally, the density of the white conveyor belt is partially sampled first to obtain the average density and determine a threshold value. The threshold value is compared with read data to detect dust and/or dirt. This dust detection operation is executed a plurality of number of times, and the number of times of detection of the dust and/or dirt at each pixel position is counted, thereby determining dust and/or dirt.
However, to obtain the average density of the conveyor belt, a memory for storing sampling data is necessary. Additionally, a line memory is also necessary to count dust and/or dirt at each pixel. The number of times of sampling is limited within the range of the number of bits of the line memory. That is, for more accurate dust detection, the memory capacity must be large.
Furthermore, in the above-described dust detection method using a conveyor belt, if the conveyor belt surface is dirty, dirt on the original glass plate surface cannot be normally detected.
As described above, the dust detection method using image data from an image reading element depends on dirt on the conveyor belt or convey roller surface of the original convey section. If not the platen glass but the conveyor belt or roller is dirty, an image is read from the image reading element as if the platen glass surface were dirty.